An optical module for converting optical signals into electric signals has a structure in which an optical device, such as semiconductor laser or photo diode, is accommodated in a case, to transmit or receive the optical signals through an optical fiber (Japanese Patent Unexamined Publications (kokai) JP-11-240741-A (1999), JP-2001-66468-A (2001)).
Among the above optical modules, a receptacle type of optical module for coupling with a connector includes an optical device 20 at the rear of an optical receptacle 70 as shown in FIG. 14, and can be connected with a plug ferrule PF of an optical connector (SC connector) in front thereof.
The above optical receptacle 70, as shown in FIG. 14, includes a fiber stub 81, a sleeve 84, a holder 85 and a sleeve case 86, in which the ferrule 82 is formed of ceramic materials, such as zirconia or alumina, and an optical fiber 83 formed of silica glass or the like is inserted and fixed in a through-hole of the ferrule 82 to obtain the fiber stub 81, and a rear end 97 of the fiber stub 81 is pressed and fixed into the holder 85, and a front end 98 thereof is inserted into an internal hole of the sleeve 84, and these are pressed or fixed with an adhesive into the sleeve case 86.
Recently, a downsized optical module is required in demand for high density assembly, and an overall length of an optical receptacle is also required to be shortened. Therefore, as shown in FIG. 15, devised is another optical receptacle 71 including: the fiber stub 81 in which the optical fiber 83 is inserted and fixed in the through-hole of the ferrule 82; the sleeve 84 for holding the plug ferrule PF connectable to the front end 98 of the fiber stub 81; and a grip ring 94 for binding free deformation of the sleeve 84, being pressed onto an outer side face 99, shown by L3, adjoining the rear end 97 of the sleeve 84. Since the grip ring 94 is pressed onto the outer side face 99 of the sleeve 84 which holds the fiber stub 81 to restrict free deformation of the sleeve 84, holding force sufficient for the fiber stub 81 can be attained even when shortening the length L2 defined as the fiber stub 81 being held by the sleeve 84 (JP-10-332988-A (1998)).
In addition, as shown in FIG. 16, further devised is another structure in which the grip ring 94 shown in FIG. 15 has a stopper portion 94a to prevent the fiber stub 81 from dropping out (JP-2003-43313-A (2003)).
As shown in FIG. 16, in a case of an optical module using the above-mentioned optical receptacle 72, a case 22 containing an optical device 20 and a lens 21 is joined by welding on the side of the rear end 97 of the fiber stub 81 of the optical receptacle 72, and the plug ferrule PF is inserted into the sleeve 84 from the front side of the optical receptacle 72 to make the optical fibers contact with each other, thereby enabling communication of optical signals.
In this case, the plug ferrule PF has an outer diameter of approximately 2.5 mmΦ for connection of SC connector, or approximately 1.25 mmΦ for connection of LC connector, with tolerance in outer diameter of ±1 μm or below, and the optical fiber provided in the through-hole thereof has an outer diameter of approximately 125 μm with tolerance in outer diameter of approximately ±1 μm as defined by JIS (Japanese Industrial Standards) or IEC (International Electorotechnical Commission) standard. Conventionally, each of components, e.g., sleeve 84, plug ferrule PF, is machined with high accuracy in order to connect cores (not shown) for transmitting optical signals to each other with low loss, the core having a disameter of approximately 10 μm, being formed in the center of the optical fiber. The sleeve 84 can hold the fiber stub 81 and the plug ferrule PF with high stability and accuracy.
Further, the front end 98 of the above fiber stub 81 is mirror finished into a curved face with a curvature radius of approximately 5 to 30 mm to reduce connection loss in contact. The rear end 97 is mirror finished with the end face tilted at approximately 4 to 10 degree together with the ferrule 82 into which the optical fiber 83 is inserted, thereby suppressing reflecting light, that is, light emitted from the optical device 20, such as laser diode, may not come back to the optical device.
In the conventional optical receptacle 72 as shown in FIG. 14, however, when adopting a technique of shortening the length L1 of the sleeve 84 and the distance L2 of front end 98 of the fiber stub 81 to respond demands of downsizing, the holding force between the sleeve 84 and the fiber stub 81 is remarkably reduced. Consequently, in a case the plug ferrule PF is loaded as tilted during connection of the plug ferrule PF to the fiber stub 81, the sleeve 84 may be also tilted, causing connection loss.
Moreover, when changing in dimension from a 2.5 mmΦ ferrule of SC connector to a 1.25 mmΦ ferrule of LC connector, the outer area of the ferrule is reduced half because of the decreased outer diameter of the ferrule. Consequently, in a case of adopting such a technique for fixing a fiber stub as the conventional technique, for example, adopting relation among the length L3 defined as the fiber stub being fixed in the holder, the outer diameter D of the fiber stub 81 and the inner diameter D1 of a fiber stub fixed portion of the holder 85, a contact area between the holder 85 and the fiber stub 81 is remarkably reduced with the fixing force significantly lowered, thereby degrading repeatability in connection loss due to motion of the fiber stub 81 during connection of optical connectors.
Furthermore, in the optical receptacles 71 and 72 as shown in FIGS. 15 and 16, even when the plug ferrule PF is loaded as tilted after the plug ferrule PF is connected to the fiber stub 81 by pressing the grip ring 94 onto the sleeve 84, it may be imagined that the sleeve 84 is hardly tilted with no connection loss. But since a load is applied to the fiber stub due to repetition of attachment and detachment of the plug ferrule PF or by a continuing spring load of the plug ferrule PF during connection, the grip ring 94 is required to effect a sufficient grip force on the fiber stub 81 and the sleeve 84, and a sufficient fixing force on the holder 85. Therefore, the sleeve 84 may be deformed or unevenly deformed during the plug ferrule PF is inserted in or extracted from the sleeve 84, resulting in unstable insertion or extraction force. These structures are not suitable for usage of attaching great importance to repeatability of attachment and detachment.
Further, the sleeve 84 may be cracked or distorted by strongly pressing in because of thin shape. In such a structure with no stopper as the grip ring 94 in FIG. 15, the grip force for tightly fixing by pressing to prevent the fiber stub 81 from dropping out must be minutely controlled.
Furthermore, since the length L2 defined as the fiber stub 81 being held by the sleeve 84 is shortened, the holding condition of the fiber stub 81 becomes unstable. Therefore, the holding condition of the fiber stub 81 by the sleeve 84 may be changed every time the plug ferrule PF becomes contact, thereby degrading repeatability of connection loss.
Recently, characteristics of variation of insertion loss and variation of reflection attenuation (wiggle characteristics) versus load applied to the plug ferrule PF perpendicularly to the optical axis thereof have been made much account of. If the length L2 defined as the fiber stub 81 being held by the sleeve 84 is shortened, the wiggle characteristics is worsened.
Further, because of the unstable holding condition, the connection portions of the optical fibers may slip mutually, causing scratches on the end face of the optical fiber 83, thereby disabling reception and transmission of optical signals.
Meanwhile, another optical receptacle in which a sleeve is integrated with a sleeve case, as shown in FIG. 17, is also known (JP-8-37461-A (1996)).
The optical receptacle 74 shown in FIG. 17 is configured of the sleeve 84 integrally formed by cutting a parent material of stainless steel or injection molding, and a ferrule stopper 93 is pressed into the cylindrical inner face.
Incidentally, in an optical receptacle requiring highly effective optical coupling with a single mode optical fiber, positioning with high accuracy is demanded even if a plug ferrule is repeatedly attached and detached. Therefore, the above-mentioned optical receptacle 74 is provided with wear resistance by forming a higher hardness film of TiC with a thickness of 1 to 10 μm using CVD, on the inner face of the sleeve 84 of stainless steel, into which the plug ferrule PF is inserted. Moreover, another optical receptacle requiring high accurate positioning and wear resistance may be made of a ceramic sleeve with wear resistance, which can be machined accurately.
As downsizing of transmitters and receivers for optical communications, generally used are optical receptacle type of optical fibers adaptable with double-core type of optical connectors having a pair for transmission and reception. Smaller optical modules are also required, and the optical modules in the transmitters and receivers for optical communications must be positioned adjacently so as to adapt to a pitch of the double-core optical connector.
As a result, in case of an optical receptacle 74T for transmitting and an optical receptacle 74R for receiving are fixed together in a metal housing 25, as shown in FIG. 18, electric signals for driving a light emitting device 20T may leaked out through the metallic optical receptacle 74T, causing noise on a light receiving device 20R, thereby degrading sensitivity for receiving.
Additionally, since electromagnetic waves may be generated by an antenna composed of the metal sleeve of the optical receptacle 74T for transmitting, a metallic shield cover must be disposed on the periphery of the transmitter and receiver for optical communications to prevent leakage of electromagnetic waves. Also, the metal sleeve of the optical receptacle 74R for receiving may pick up an external noise, thereby degrading sensitivity for receiving. Noise characteristics thereof must be improved.